Certain aqueous coating compositions can be applied by the process of electrodeposition. Electrodeposition is the process whereby a coating composition is placed in contact with an electrically conductive anode and an electrically conductive cathode with the surface to be coated being either the anode or the cathode. Following contact with the coating composition, an adherent electrically insulating film is deposited on one of the electrodes when a sufficient voltage is impressed between the electrodes. Electrodepositable coating compositions can be applied to a variety of electroconductive substrates especially metals such as steel, aluminum, copper, magnesium) and conductive carbon-coated materials.
Electrodepositable coating compositions which are applied cathodically (that is, the surface to be coated is the cathode) are preferred to those which are applied anodically because cationic compositions usually provide superior corrosion resistance. Electrodepositable coating compositions based on epoxy-derived resins which are crosslinked with materials containing blocked isocyanate groups upon application of heat are very cox,]on in the automotive industry.
Often, a topcoat system is subsequently applied to electrodeposited, cured coatings in order to achieve even greater corrosion resistance and durability than the electrodeposited coating alone would provide. Additionally, topcoat systems are applied in order to provide an attractive, glossy finish which must match a specific color standard. This is especially true in the automotive industry, where, for example, a non-conductive plastic part which has not been electrocoated abuts a metal part which has been electrocoated, and the color of these parts must match.
Thermosetting topcoat systems which contain acrylic, polyester and/or alkyd resins that crosslink with aminoplast or blocked polyisocyanate curing agents are very common in the automotive industry. These topcoats can be spray-applied as a single color coat or as a two-coat base/clear system. It is well known in the art that certain light-colored topcoat systems such as whites, light blues, creams or tans are subject to yellowing when they are applied over certain cured electrodepositable coating compositions. Such yellowing can be caused by various factors such as the electrodeposition coating composition, the topcoat composition or the chemical composition of the atmosphere in which the coating compositions are cured, Yellowing which is related to the electrodeposition coating composition will hereinafter be referred to as "ED-related yellowing."
ED-related yellowing is especially pronounced when low molecular weight aromatic blocked isocyanate curing agents are used in the electrodepositable coating composition. Aliphatic blocked isocyanate curing agents produce electrodepositable coating compositions which cause less ED-related yellowing in the subsequently applied topcoat systems, but these curing agents are generally more expensive than certain aromatic blocked isocyanate curing agents such as toluene diisocyanate (TDI). Aromatic blocked. isocyanate curing agents which have higher molecular weight than TDI also produce electrodepositable coating compositions which tend to cause less ED-related yellowing in the subsequently applied topcoat systems.
The present invention is based on the discovery that blocked isocyanate curing agents which contain oxazolidone groups in addition to blocked isocyanate groups produce very little ED-related yellowing in subsequently applied topcoat systems. This allows formulation of non-yellowing or reduced yellowing electrodepositable coating compositions with a wide variety of blocked isocyanate curing agents, particularly blocked aromatic isocyanate curing agents based on relatively low molecular weight aromatic isocyanates such as TDI, which are non-yellowing.